1. Field of the Invention (Technical Field)
The present invention relates to magnets for nuclear magnetic resonance applications, more specifically to magnets that generate at least one region with a relatively homogenous magnetic field at a position that is remotely located from the magnet assembly.
2. Background Art
A major use of magnets generating regions of uniform magnetic fields is in magnetic resonance. Nuclear magnetic resonance (xe2x80x9cNMRxe2x80x9d) is a well-established technique with applications fields such as physics, chemistry, mechanical engineering, civil engineering, nuclear engineering, petroleum engineering, food processing, pharmaceutical production, biology, and medicine. Most applications of NMR use a magnet to generate a uniform magnetic field. For superconducting magnets having a cylindrical bore, the uniform field region is within the bore whereas, for resistive electromagnets, the uniform field region is between the magnet""s poles. In either configuration, a sample to be studied is confined to a uniform field region that is small compared to a characteristic dimension of the magnet.
Few NMR applications rely on xe2x80x9cexternalxe2x80x9d or xe2x80x9cremotexe2x80x9d uniform field regions in which the objects being examined are positioned to one side or outside of the NMR apparatus. U.S. Pat. No. 3,019,383 (xe2x80x9c""383 Patentxe2x80x9d) discloses an early remote apparatus for detecting subsurface liquids, such as water, from a position on the surface while using only the magnetic field of the Earth. A. G. Semenov, M. D. Schirov, A. V. Legchenko, A. I. Burshtein, and A. J. Pusep, further developed this approach in the 1980""s (UK Patent Application GB2198540A, xe2x80x9cDevice for measuring parameters of underground mineral deposits,xe2x80x9d filed May 29, 1986) with a more practical apparatus. Like the apparatus described in the ""383 Patent, the apparatus of Semenov et al., does not use a man-made magnet; it relies solely on the NMR signal from protons in subsurface liquids processing in the weak magnetic field of the Earth.
NMR based solely on the Earth""s magnetic field inherently yields a low signal-tonoise ratio (S/N), unless the sample size and corresponding number of nuclei are very large. S/N is an essential parameter in NMR; it is directly proportional to the number of atomic nuclei and to approximately the {fraction (3/2)} power of the magnetic field strength. Stronger magnetic fields align more spins and also make the spins precess faster, which leads to an increase in signal magnitude. NMR applications using the Earth""s magnetic field overcome weak magnetic field limitations by examining a very large sample with a correspondingly large number of-atomic nuclei. A typical magnet used for NMR has a magnetic field strength that ranges from a fraction of Tesla to tens of Tesla. In contrast, the Earth""s magnetic field strength is typically around xc2xd Gaussxe2x80x94approximately 100,000 times weaker than most commercially available NMR magnets. Applying the {fraction (3/2)} power criterion to the difference in magnitude between the Earth""s field and commercial field magnet strengths equates to an Earth produced signal that is approximately 3xc3x97107 less than that of a commercial system for the same sample size. To compensate for the low signal strength, the apparatus of Semenov et al. uses a circular coil having a diameter of 100 meters that is placed on the ground to detect subsurface signals. The volume of the sample thus examined is approximately equal to a hemisphere having a diameter of 50 meters, equivalent to 3.3xc3x971010 cubic centimeters. When compared to an ordinary laboratory NMR sample of a few cubic centimeters, this represents an increase of 1010. Such an increase is sufficient to obtain useable signals despite the many adverse conditions of performing in situ environmental NMR.
The aforementioned example demonstrates the difficulty of performing subsurface or otherwise equivalent NMR experiments using Earth""s field-based techniques for small sample volumes, such as those encountered when examining shallower depths (less than 50 m) that implies much smaller sample volumes. Therefore, a need exists for a NMR apparatus to perform experiments for remote regions having xe2x80x9cmid-rangexe2x80x9d sample volumes between the xe2x80x9csmall rangexe2x80x9d (sub- to a few cubic centimeter dimensions of the laboratory apparatus) and the xe2x80x9clarge rangexe2x80x9d (millions of cubic centimeter dimensions of the Earth""s field apparatus).
To meet the mid-range need, several classes of remote NMR have been developed that generate a magnetic field that is significantly stronger than the Earth""s field. One subclass of such NMR apparatuses comprises magnets that are designed to fit inside holes to observe NMR signals from samples outside the hole. The most common application of this subclass is NMR downhole oilwell logging. Early versions of this subclass (as disclosed in U.S. Pat. No. 3,213,357, to Brown) used a NMR logging tool having a coil that was temporarily energized to create a magnetic field in the formation surrounding the bore hole. This field acted to orient, that is, to prepolarize, the spins belonging to the fluid of interest such as oil or water. The nuclear spins of the water around the bore hole were detected as they processed around the Earth""s magnetic field immediately after the prepolarization field was turned off. Thus, this method used the applied magnetic field to prepolarize the spins and then used the weaker Earth""s field to conduct the remainder of the experiment. The use of the prepolarizing field ameliorates the signal loss at the lower fields from the {fraction (3/2)} power dependence alluded to earlier; nevertheless, signal loss will still be incurred approximately at the xc2xe power of the field strength.
Another scheme emerged in the 1980s, one that placed permanent magnets downhole to generate a magnetic field stronger than the Earth""s field as disclosed in U.S. Pat. No. 4,350,955 to Jackson, et al. (xe2x80x9c""955 Patentxe2x80x9d). The ""955 Patent describes a NMR apparatus with a magnet assembly having two cylindrical permanent magnets aligned along the bore hole axis. The apparatus of the ""955 Patent projects a thin annular region of substantially uniform magnetic field outside the bore hole. Such schemes use both prepolarization and detection in the presence of magnetic fields stronger than Earth""s field, about 0.5 Gauss, which gives rise to much better S/N. The {fraction (3/2)} power dependence of the signal on the field strength means that even a 5 Gauss field results in a 32-fold gain in S/N whereas a 50 Gauss field yields a factor of 1000 over S/N in the Earth""s magnetic field and a 500 Gauss field leads to a gain of more than 30,000.
The ""955 Patent states at col. 2, II. 64-68; col. 3, II. 1-11: xe2x80x9cA well known requirement for generating an observable NMR signal is a relatively homogeneous magnetic field across a sample volume in order that the precessional frequencies of the nuclei within the sample will be relatively uniform. Previous attempts to xe2x80x9cfocusxe2x80x9d a region of NMR sensitivity into the formation using a non-uniform field (e.g., a magnetic dipole) which decreases rapidly and monotonically with increasing distance from the axis suffer from the fact that the radial width of a sample volume within which the magnetic field homogeneity is good enough to support NMR is extremely small.xe2x80x9d
This paragraph appears in the ""955 Patent to Jackson, which the present application incorporates by reference.
U.S. Pat. No. 4,710,713 (xe2x80x9c""713 Patentxe2x80x9d), entitled xe2x80x9cNuclear Magnet Resonance Sensing Apparatus and Techniquesxe2x80x9d, to Strikman, issued Dec. 1, 1987, is an alternative scheme for bore hole NMR logging. This patent discloses a nuclear magnetic resonance sensing apparatus having one or more magnets to generate a magnetic field in a region remote therefrom. The one or more magnets define a longitudinal axis and the static field created by the magnets has a field direction substantially perpendicular to the longitudinal axis.
U.S. Pat. No. 4,717,876 (xe2x80x9c876 Patentxe2x80x9d), entitled xe2x80x9cNMR Magnet System for Well Logging,xe2x80x9d to Masi et al., issued Jan. 5, 1988, describes an apparatus that is an improvement to that of the ""955 Patent. The ""876 Patent discloses a magnetic structure producing a substantially uniform toroid field in a region external to the magnetic structure. The first and second order partial derivatives with respect to the spatial coordinates are equal to zero at any selected point with in the uniform toroidal field. The magnetic structure comprises at least first magnet means magnetized in a direction extending along a first predetermined axis and a second magnet means magnetized in at least one direction extending at an angle to first said axis. In one embodiment, the first magnet means consists of a pair of elongated magnets. This magnet pair is axially aligned along the first axis but spaced from one another, each of the magnets having an end space defining a predetermined pole. The magnet pair is arranged so that corresponding pole faces are located opposite of each other. The second magnet means of this embodiment consists of a second pair of magnets, wherein each of the magnets is disposed adjacent to the end face of the aforementioned first magnets. The second magnets are magnetized in directions extending radially to the first axis.
U.S. Pat. No. 4,717,877 (xe2x80x9c""877 Patentxe2x80x9d), entitled xe2x80x9cNuclear Magnet Resonance Sensing Apparatus and Techniquesxe2x80x9d to Taicher et al., issued Jan. 5, 1988, is yet another bore hole NMR patent, generally similar in geometry to the apparatus of the ""713 Patent. This patent discloses a nuclear magnetic resonance sensing apparatus having one or more magnets to generate a static magnetic field that decays with distance in a region remote therefrom. The one or more magnets define a longitudinal axis and the static field direction is perpendicular to this axis.
U.S. Pat. No. 4,717,878 (xe2x80x9c""878 Patentxe2x80x9d), entitled xe2x80x9cNuclear Magnetic Resonance Sensing Apparatus and Techniques,xe2x80x9d to Taicher, et al., issued Jan. 5, 1988, is another patent describing a bore hole NMR apparatus, with geometry generally similar to the ""713 Patent. The ""878 Patent discloses a nuclear magnetic resonance sensing apparatus having a magnet assembly that includes at least three magnets. The at least three magnets are arranged along a longitudinal axis in side-to-side arrangements and have a generally uniform cross-section which, when joined together, define a circular cross-section. The at least three magnets are magnetized in directions extending perpendicular to the longitudinal axis. At least one of the magnets has a magnetization direction opposite to the magnetization direction of at least one of the other magnets. There are two regions of interest for this geometry comprising two long bar-shaped regions that are located on opposite sides of the bore hole and extended along the bore hole.
The ""878 Patent to Taicher et al. stater (col. 6, II. 3-5) that the disclosed device causes xe2x80x9cthe one or more magnets to generate a static magnetic field of generally uniform amplitude in a remote region containing materials sought to be analyzedxe2x80x9d.
This phrase appears in the ""878 Patent to Taicher et al., which the present application incorporates by reference.
U.S. Pat. No. 4,933,638 (xe2x80x9c638 Patentxe2x80x9d), entitled xe2x80x9cBore Hole Measurement of NMR Characteristics of Earth Formations, and Interpretations Thereofxe2x80x9d to Kenyon et al., issued Jun. 12, 1990, and U.S. Pat. No. 5,055,788 (xe2x80x9c""788 Patentxe2x80x9d), entitled xe2x80x9cBore Hole Measurement of NMR Characteristics of Earth Formations,xe2x80x9d to Kleinberg et al., issued Oct. 8, 1991, describe schemes for bore hole well logging. The apparatuses of these patents are designed to produce a static and relatively homogenous magnetic field in a volume adjacent to and on one side of the apparatus. At least two slab-shaped magnets are used to generate the homogenous field that is outside the borehole and extends along one side of it. In several embodiments a third slab-shaped magnet is used that is interposed between the two slab-shaped magnets, in an analogous fashion to the ""878 Patent. Depending on the desired field characteristics, the polarity of the interposed magnet is aligned with the other two magnets or in opposition to that of the other two magnets. The position of the interposed magnet with respect to the other two magnets is also used to affect the characteristics of the relatively homogenous external field. These patents also disclose a method and apparatus for generating a transverse radio frequency field within the homogenous external static magnetic field volume.
U.S. Pat. No. 5,629,623 (xe2x80x9c""623 Patentxe2x80x9d), entitled xe2x80x9cPulse Nuclear Magnetism Tool for Formation Evaluation While Drilling,xe2x80x9d to Sezginer et al., issued May 13, 1997 is for an apparatus that allows for making NMR measurements while a bore hole is being drilled. This patent discloses a sleeve-shaped tubular magnet that is magnetized transversely to the axis of the drilling tool. The length of the magnet is approximately 2 to 6 feet.
All of the aforementioned methods and apparatuses for bore hole logging are designed to detect protons at a distance of a few centimeters to several centimeters past the edge of the bore holexe2x80x94the bore hole having typically a diameter of approximately 20 cm.
U.S. Pat. No. 5,572,132 (xe2x80x9c""132 Patentxe2x80x9d), issued Nov. 5, 1996, to Pulyer, et al., discloses an apparatus for biomedical or industrial applications having a magnetic resonance imaging probe that generates its own magnetic field. The probe comprises a primary magnet having a longitudinal axis and an external surface extending in the axial direction. The primary magnet generates a field that provides a region of substantial homogeneity of axial directed field components along the surface and proximate to the surface. In one embodiment, the field is generated by two magnets spaced axially and having axial alignment, wherein the magnetic polarization of each magnet is oriented in the same direction. In another embodiment, the primary magnet is a cylindrical permanent magnet.
U.S. Pat. No. 5,757,186 (xe2x80x9c""186 Patentxe2x80x9d), entitled xe2x80x9cNuclear Magnetic Resonance Well Logging Apparatus and Method Adapted for Measurement While Drilling,xe2x80x9d to Taicher et al., issued May 26, 1998. This patent discloses a nuclear magnetic resonance sensing apparatus comprising a magnet for inducing a static magnetic field that is radial, rotationally symmetric, and perpendicular to the longitudinal axis of the magnet. In one embodiment, the magnet comprises a plurality of axially magnetized cylinder segments. A particular feature of the axially magnetized cylinders is that the magnetization of each cylinder is proportional in magnitude to its axial distance from the center plane of the magnet. The magnetization of the segments is directed towards the center plane. In an alternative configuration of the magnet, the magnet is constructed from a plurality of radial segments of a cylinder which, when assembled, form a substantially cylindrical annular magnet. In this embodiment, each segment is magnetized radially, along the length perpendicular to the axis of the cylindrical annular magnet.
Unilateral or unidirectional magnets for remote NMR apparatuses comprise another class. Apparatuses of this class are used to examine a sample positioned to one side of the magnet. A distinction is made between unilateral or unidirectional instruments and those NMR apparatuses using magnets that project a magnetic field to only one side of the bore hole (e.g., ""638, ""788, and ""623 Patents). Unilateral or unidirectional magnets are designed to work on a planar surface and project a uniform magnetic field region to the other side of the planar surface. Unilateral apparatuses are well suited for NMR applications requiring mobility along a surface. Mobile detection NMR applications in fields and on bridges require use of a portable magnet in conjunction with a NMR excitation and detection coil placed on the surface of the system being studied. Such apparatuses differ from the down-hole concept due to different geometric constraints. In the down-hole geometry, only the length along the bore hole is not severely burdened by geometrical limitations thus the length can be made significantly longer than the bore-diameter. Increasing the length along the bore hole axis acts to elongate the uniform field region rather than increasing the magnetic field strength at a specific uniform field region.
Early applications by Southwest Research Institute, including moisture detection for farm fields, moisture monitoring for drying of concrete, and remote detection of land mines, are reported by J. D. King, W. L. Rollwitz, and G. A. Matzkanin, xe2x80x9cNuclear Magnetic Resonance Techniques for Explosives Detection, Final Report (Contract No. DMK02-74-C-0056) prepared for U.S. Army Mobility Equipment Research and Development Center, Fort Belvoir, Va. 22060, July, 1975. Many of these applications use a main electromagnet in the shape of an upside down xe2x80x9cUxe2x80x9d with an approximate linear dimension of one meter and a NMR RF xe2x80x9cpancake coilxe2x80x9d located near the surface of the region of study and in the open part of the xe2x80x9cU.xe2x80x9d The depth of investigation of these apparatuses is limited to few centimeters.
G. Eidmann, R. Savelsberg, P. Blxc3xcmler, and B. Blxc3xcmich, xe2x80x9cThe NMR MOUSE, a Mobile Universal Surface Explorer,xe2x80x9d J. Magn. Reson. A122, 104-109 (1996) describe a small analog of the above apparatus of King et al., known as xe2x80x9cNMR MOUSE,xe2x80x9d developed by Blxc3xcmich and collaborators. The magnetic field is made non-uniform in the region of interest in order to isolate a NMR signal from a particular thin shell of nuclear spins; however, this limits S/N.
Some of the aforementioned unilateral NMR apparatuses of King, et al. and the NMR MOUSE apparatus by Eidmann, et al. have magnetic fields, xe2x80x9cB,xe2x80x9d that may have local maxima close to the magnet but then become progressively weaker with respect to increasing distance from the magnet. Such a design allows the operator to choose an appropriate frequency xe2x80x9cfxe2x80x9d that specifies a surface on which the NMR signal originates, that is, where magnetic field strength obeys the Larmor equation B=(2xcfx80/xcex3)f, where xcex3 is the gyromagnetic ratio. For a monotonically decreasing magnetic field, the Larmor equation is satisfied only within a thin shell. If the bandwidth of the RF pulse in NMR is approximately 1 kHz or less, the shell, in which the NMR signals originate, spans field values that differ only by 0.25 G for protons. If the field drops from 5000 G to zero over 5 meters, the average gradient is 10 G/cm so the shell is only approximately 0.25 mm thick, assuming a 1 kHz RF pulse. In this particular scheme without a uniform field region, a NMR signal arises from an oddly shaped xe2x80x9cspin resonantxe2x80x9d shell.
King et al. (1975) also discloses designs for multiple magnet element assemblies that project a uniform field to one side. One such design consists of two identical circular loops of wire coaxially displaced from each other and carrying electrical currents in opposite directions. The magnetic fields from the two loops cancel differently at different distances along the cylindrical axis and result in a substantially uniform field region on axis just outside each loop. Thus, the major benefit of a uniform field region, a sweet spot, is to provide a well-defined sensitive volume rather than a thin shell from which NMR signals can be obtained.
Another scheme by King et al. (1975) is a pair of coplanar rectangular loops, parallel to each other and spaced apart by approximately a loop dimension. When equal but opposite currents flow in the two side-by-side loops, the magnetic field at points on a perpendicular bisector of the space between the coplanar loops is parallel to the plane and exhibits a maximum as a function of the distance away from the plane, i.e., there will be a region of substantially uniform field at some distance from the plane. In both of these examples from King, et al. (1975), the distance to the position of substantially uniform field is a small fraction of the extent of the magnet in the direction perpendicular to the projection direction.
U.S. Pat. No. 4,721,914 (xe2x80x9c""914 Patentxe2x80x9d), entitled xe2x80x9cApparatus for Unilateral Generation of a Homogenous Magnetic Field,xe2x80x9d to Fukushima et al., issued Jan. 26, 1988, discloses an apparatus for generating a homogenous static magnetic field by two circular electromagnetic coils. The currents in the coils are in opposition so that the resulting magnetic fields subtract. In all embodiments the radius of the fore coil is smaller than the radius of the aft coil. The design of the two coils provides that for a homogenous static field region external to the coil wherein both the first and second derivatives of the field with respect to the coil axis are nulled.
U.S. Pat. No. 5,095,271 (xe2x80x9c""271 Patentxe2x80x9d), entitled xe2x80x9cCompact Open NMR System for In Situ Measurement of Moisture, Salinity, and Hydrocarbons,xe2x80x9d to Ohkawa, issued Mar. 10, 1992, discloses a nuclear magnetic resonance device having a cylindrical magnet that has a plurality of south poles and an equal plurality of north poles, the poles being dimensioned and disposed on the magnet to establish a toroidal zone external to the magnet wherein the field of the magnet is of substantially constant strength. The direction of the field is orthogonal to the cylinder axis and the uniform field region is located radially from the center of the magnet.
U.S. Pat. No. 5,382,904 (xe2x80x9c""904 Patentxe2x80x9d), entitled xe2x80x9cStructured Coil Electromagnets for Magnetic Resonance Imaging and Method for Fabricating the Same,xe2x80x9d to Pissanetzky, issued Jan. 17, 1995, discloses electromagnets suitable for use in NMR imaging that are constructed according to a structured coil methodology. The magnet construction results in a plurality of coils of varying current polarities and of irregular shape and size that are optimized to provide a uniform field. The uniform field is located internal to the coil structure or optionally outside the location of the coils. The electromagnets developed according to this method comprise at least one coil, wherein the current conducted at a plurality of locations therewithin is determined by the method. In several embodiments the magnet comprises coil assemblies having several individual coils carrying currents in clockwise and counter-clockwise directions. In at least one embodiment, pairs of such coil assemblies are used. All embodiments envisage the use of superconducting electromagnets. In a typical embodiment of a remote NMR magnet, a superconducting winding of radius 125 cm and thickness 80 cm projects a uniform field region (to 10 ppm) that is a 10 cm radius sphere whose center is 15 cm from the nearest coil. The magnet shall have six separate coil windings within the dimensions given and the average current density required to project a xc2xd Tesla field in the uniform field region is 1700 A/m2.
U.S. Pat. No. 5,744,960 (xe2x80x9c960 Patentxe2x80x9d), entitled xe2x80x9cPlaner Open Magnet MRI System,xe2x80x9d to Pulyer, issued Apr. 28, 1998, also discloses a magnetic resonance imaging system having an open magnet structure. The open magnet structure has a primary magnet with two primary pole pieces connected by a primary ferromagnetic core for creating an internal flux. A bias magnet having polarity opposite to the primary magnet is located between the two primary pole pieces. Superposition of the magnetic fields of the primary magnet and the bias magnet provide for a relatively homogenous field external to the primary and bias magnets.
In summary, the present state of art consists of several distinct classes of NMR apparatuses. One class consists of conventional magnets having useable field regions that are bounded or surrounded by the magnet components and not external to it. The second class is that of remote or external magnets in which the useful magnetic field is located outside the magnet assembly. There are two subclasses of such remote magnets. In the first subclass, there is no substantially uniform field region that is useful. The second subclass of remote magnets consists of magnets that have substantially uniform field regions.
Yet another geometrical classification within the external or remote magnet class comprises external or remote magnets that are intended to work inside a hole such as a geological bore hole or inside a vessel in an intact animal. One group of these magnets projects a useful magnetic field omni-directionally in at least in two directions outside the hole or vessel while another group projects the useful field primarily in a single direction.
The present invention is of a magnetic field generating apparatus having a body with a substantially annular cross-section. The body of this embodiment has at least one diameter, a longitudinal axis and a finite length. The body comprises a magnet system for producing a substantially uniform magnetic field in a region external to the body. In this preferred embodiment, the uniform field region is substantially centered on the longitudinal axis and positioned at a distance of at least 0.15 times a largest of the body""s diameters from the body. The magnet system of this preferred embodiment comprises electronic magnets and/or permanent magnets. Commercially available permanent magnets comprise a variety of shapes and sizes. A plurality of individual permanent magnets is suitable for creating the magnet system. Individual permanent magnets are configurable in a variety of configurations, including, but not limited to, a plurality of permanent magnets forming a stack. A plurality of magnet stacks is positionable in a body to form a magnet system. This particular embodiment of the present invention, when the magnet system comprises permanent magnets, uses a plurality of magnet stacks configured in a body to form a substantially annular magnet system. In this preferred embodiment, annular refers generally to the overall configuration of the apparatus whether or not the body comprises a continuous annulus or a discontinuous annulus. In any instance, placement of at least one annular steel end piece onto the body generally acts to enhance the magnet system azimuthal field homogeneity. The magnet system of this embodiment is shortable by addition of shorting means comprising, but not limited to, ferromagnetic annular cylinders and ferromagnetic cylinders. Such shorting means are placed concentrically to the magnet body, either internally or externally. This embodiment comprises part of a NMR device through addition of a coil system for producing an oscillating magnetic field substantially transverse to the uniform magnetic field. Such a coil system is useable either alone or in conjunction with addition of a coil system for producing a rotating magnetic field substantially transverse to the uniform magnetic field. The coil system for producing a rotating magnetic field is useable either alone or in conjunction with a coil system for producing an oscillating magnetic field. Such coil systems are also useful for detection of electromagnetic radiation.
The present invention is also of a magnetic field generating apparatus comprising at least two concentric bodies having substantially annular cross-sections, each having at least one diameter and all having a substantially common longitudinal axis. These bodies have finite lengths. The bodies also have magnet systems for producing a substantially uniform magnetic field in a region external to the bodies and substantially centered on the longitudinal axis. The substantially uniform field region is positioned at a distance of at least 0.15 times a largest of the diameters from the bodies. Variations of the magnet system and shorting means of the aforementioned embodiment also apply to this particular embodiment. Such variations include, but are not limited to, magnet systems comprising electronic magnets and/or permanent magnets. A particular variation of this embodiment comprises means for moving at least one of the concentric bodies relative to another of the concentric bodies. Such means allow for adjustability of characteristics of the substantially uniform field region. Another particular variation of this embodiment features at least one of electronic magnet and current adjustment means for adjusting current of the at least one electronic magnet system. This variation, as other variations, is useful in conjunction with other variations of this embodiment and other embodiments of the present invention. This particular embodiment comprises part of a NMR device through addition of a coil system for producing an oscillating magnetic field substantially transverse to the uniform magnetic field. Such a coil system is useable either alone or in conjunction with addition of a coil system for producing a rotating magnetic field substantially transverse to the uniform magnetic field. The coil system for producing a rotating magnetic field is useable either alone or in conjunction with a coil system for producing an oscillating magnetic field. Such coil systems are also useful for detection of electromagnetic radiation.
The present invention is further of a magnetic field generating apparatus comprising a body having a substantially annular cross-section having at least one diameter and a longitudinal axis. The body also comprises a finite length. Additionally, this embodiment comprises a first magnet system for producing a substantially uniform magnetic field in a region external to the body and substantially centered on the longitudinal axis and a second magnet system comprising a cylinder substantially positioned on the longitudinal axis. Positioning of the cylinder along the longitudinal axis alters the substantially uniform magnetic field. Variations of the magnet system and shorting means of aforementioned embodiments also apply to this particular embodiment. Such variations include, but are not limited to, magnet systems comprising electronic magnets and/or permanent magnets. In this particular embodiment, a second magnet system comprises a magnet direction parallel to the first magnet system or anti-parallel to the first magnet system. This embodiment comprises part of a NMR device through addition of a coil system for producing an oscillating magnetic field substantially transverse to the uniform magnetic field. Such a coil system is useable either alone or in conjunction with addition of a coil system for producing a rotating magnetic field substantially transverse to the uniform magnetic field. The coil system for producing a rotating magnetic field is useable either alone or in conjunction with a coil system for producing an oscillating magnetic field. Such coil systems are also useful for detection of electromagnetic radiation.
The present invention is additionally of a magnetic field generating apparatus comprising at least two bodies having substantially annular cross-sections, each having at least one diameter and all having a substantially common longitudinal axis. The bodies also have finite lengths. This embodiment further comprises magnet systems for producing at least one substantially uniform magnetic field in a region external to the bodies and substantially centered on the longitudinal axis. In this embodiment, the at least one substantially uniform field region is substantially centered between the at least two bodies which are separated by a distance of at least 0.15 times a largest of said diameters. Variations of the magnet system and shorting means of aforementioned embodiments also apply to this particular embodiment. Such variations include, but are not limited to, magnet systems comprising electronic magnets and/or permanent magnets. This particular embodiment comprises part of a NMR device through addition of a coil system for producing an oscillating magnetic field substantially transverse to the uniform magnetic field. Such a coil system is useable either alone or in conjunction with addition of a coil system for producing a rotating magnetic field substantially transverse to the uniform magnetic field. The coil system for producing a rotating magnetic field is useable either alone or in conjunction with a coil system for producing an oscillating magnetic field. Such coil systems are also useful for detection of electromagnetic radiation.
Aforementioned embodiments and variations thereof comprise part of a NMR device when appropriately equipped as described herein. When apparatuses of the present invention are used as a NMR device the present invention also comprises an inventive method of nuclear magnetic resonance. The invention when used for NMR, is useful for measuring NMR signals from all known nuclei having magnetic resonance properties, including, but not limited to, isotopes of hydrogen, nitrogen, carbon, fluorine, oxygen, phosphorous, sulfur, and chlorine.
The present invention is also of a method comprising: introducing a body proximate to a volume of interest, the body comprising an annular cross-section having a longitudinal axis substantially collinear with the volume of interest, a finite length, and a magnet system for producing a substantially uniform magnetic field in the volume of interest; producing at least one magnetic field component substantially transverse to the uniform magnetic field wherein the at least one magnetic field component is selected from a member of the group consisting of oscillating magnetic field and rotating magnetic field; and measuring an electromagnetic response from the volume of interest. The method is useful with aforementioned apparatus embodiments and variations thereof. The method can also incorporate a producing step that defines a region within the volume of interest wherein the measuring step measures an electromagnetic response from the defined region within the volume of interest. When defining a region within a substantially uniform field, the producing step for defining a region within the volume of interest comprises producing a first magnetic field component substantially transverse to the uniform magnetic field and producing a second magnetic field component substantially transverse to the uniform magnetic field wherein the first magnetic field component spatially differs from the second magnetic field component. For NMR applications, the aforementioned first and second magnetic field components vary with time. In other NMR applications, at least one additional magnetic field is used to provide, for example, a magnetic field gradient in the uniform field region. In some NMR applications, such at least one additional magnetic field varies with time. In a preferred embodiment of the method, the first magnetic field component is substantially orthogonal to the second magnetic field component. Generally, the first magnetic field comprises a field distribution substantially different than that of the second magnetic field.
The methods and/or apparatuses of the present invention are useful for examining a volume of interest that is subterranean, a biological material, and/or a building material. For example, the methods and apparatuses of the present invention are useful when the volume of interest lies within an animal, a plant, the Earth, a building, a road, an airport runway, a statue, a pillar, a commercial good, and/or a food where such food is, for instance, a fruit. The methods and apparatuses of the present invention is useful for detecting NMR signals from these materials as well as others. These examples are, of course, not exhaustive of the usefulness of the present invention because other uses exist in physics, chemistry, mechanical engineering, civil engineering, nuclear engineering, petroleum engineering, food processing, pharmaceutical production, biology, medicine, and allied fields. Furthermore, the methods and/or apparatuses are useful for examining a volume of interest wherein a partition lies between the volume of interest and the body.
Accordingly, a primary object of the present invention is to project a region of substantially uniform magnetic field in a specific direction at some distance from a magnetic body. In a preferred embodiment of the present invention, a permanent magnet serves as the magnetic body and comprises a right circular hollow cylinder with one end of the cylinder being a north magnetic pole and other end being south magnetic pole. The uniform magnetic field region of this preferred embodiment lies at a local maximum of the field that is located along a cylinder axis of the magnetic body, or magnet. There is a relationship between the relative distance from the magnet to the uniform field region versus the intensity of the magnetic field at the uniform field region. With currently available magnetic materials such as neodymium iron boron (NdFeB), a magnetic field intensity of the order of several hundred Gauss is obtainable at a uniform field region and local maximum that is at a distance approximately equal to the radius of the magnet beyond the end of the magnet.
A primary advantage of the present invention over the prior art is that the distance to the uniform field region in relation to the transverse dimension of the magnet is significantly larger; in other words, the present invention achieves a large aspect ratio, defined as the ratio of the distance to the uniform field region to the magnet""s cross-sectional distance, when compared to prior art.
Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.